<head>
<script src="https://unpkg.com/react@17/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@17/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/@babel/standalone/babel.min.js"></script>
</head>
<body>
<div id="root"></div>
<script type="text/babel">
  const Calculator=()=>{
    const [a, setA] = React.useState(0)
    return <div>
        <h1>计算器</h1>
        <input name="a" onChange={()=>{}} value={a}/>
        <select>
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
        </select>
        <input name="b" />
        <button>=</button>
        <span>???</span>
    </div>
  }
  ReactDOM.render(
    <Calculator/>,
    document.getElementById('root')
  );
</script>
</body>
